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In the spray-dried process, the feed to the spray dryer is a mixture of concentrated aroma and hydrolyzed fractions, with the preserved aroma components added. In order to maximize aroma retention, drying of the extract takes place under conditions that maintain low powder temperatures. Different types of spray dryers can be used for drying of coffee. Bulk density and colour control is possible by means of in-line gas mixers. Inert gas is injected into the feed system just prior to the nozzle atomizer used in the spray drying system.

In the spray-dried process, the water is allowed to evaporate, forming a concentrate. The concentrated coffee is sprayed from a high tower in a large hot-air chamber. As the droplets fall, the remaining water evaporates. Dry crystals of coffee fall to the bottom of the chamber which in turn is collected in storage bins. The high temperatures involved in this method tends to affect the oils of the coffee, which leads to loosing of more flavour when compared to Freeze dried coffee.

In cases where spray dried powders require further agglomeration, an additional process stage is used involving powder wetting, after drying and cooling. Control of wetting is carried out with water and/or saturated steam in an agglomeration chamber equipped with a rotating impactor. The agglomerates are then dried and cooled in the attached fluid bed, followed by sieving and packing. Fines and oversize fractions are reprocessed within the agglomeration plant.

The freeze-drying method preserves the ‘coffee flavour’ to a great extent. In this process, first, the coffee is allowed to sit so the water evaporates naturally, leaving a concentrated coffee solution. This concentrate is then frozen to around – 40 Degree Celsius. The remaining water freezes into ice crystals. Sublimation (a natural process similar to evaporation) is used to remove the ice, leaving behind dry grains of coffee.

In this process, agglomerated wet coffee granules are frozen. This is a very important stage. Freezing too fast leads to large ice crystals and a very porous product and can also affect the colour of the coffee granules. Thereafter, frozen coffee is placed in the drying chamber, in metal trays. A vacuum is created within the drying chamber. The strength of the vacuum is critical to the speed of the drying and ultimately, to quality of the product. Great care must be taken to produce a vacuum of suitable strength.

Thereafter, the drying chamber is warmed, most commonly, by radiation. However, conduction is used in some plants and convection has been proposed in some small pilot plants. A possible problem with convection is uneven drying rates, within the chamber, which would give an inferior product.

Condensation – The previously frozen water in the coffee granules expands to 107 of its volume, The removal of this water vapour from the chamber is vitally important, making the condenser the most critical and expensive components in a freeze drying plant.

The freeze dried granules are removed from the chamber and packaged.